Magnetic head

ABSTRACT

A magnetic sensor is constructed to be capable of detecting the change of tunnel current due to co-tunneling effect at a high S/N ratio by using a tunneling magneto-resistive element having a first magnetic layer of a soft magnetic material formed on a flat substrate, first and second tunnel barrier layers formed on the first magnetic layer, magnetic particles of a ferromagnetic material provided between the first and second tunnel barrier layers, and a second magnetic layer of a soft magnetic material formed on the second tunnel barrier layer so as to create tunneling junctions.

This is a continuation of U.S. patent application Ser. No. 09/134,458,filed Aug. 14, 1998, now U.S. Pat. No. 6,232,777.

BACKGROUND OF THE INVENTION

The present invention relates to a tunneling magnetoresistive element,and a magnetic sensor using the same.

A remarkable magnetoresistance effect is caused in a small doublejunction system, and a tunneling magnetoresistive element (TMR) of thesmall double junction type is proposed according to a known example 1(K. Ono, Hiroshi Simada, and Youiti Ootuka, “Journal of the PhysicalValve Effect and Magneto-Coulomb Oscillations,” Journal of the PhysicalSociety of Japan, vol. 66, no. 5, May 1977, pp. 1261-1264).

FIGS. 12A and 12B show the conventional tunneling magnetoresistiveelement of the known example 1. FIG. 12A is a plan view, and FIG. 12B isa cross-sectional front view. As shown in FIGS. 12A and 12B, softmagnetic layers (electrodes) 200, 201 of Ni are formed on the surface ofan insulating layer 10. In addition, tunnel barrier layers (oxide filmbarriers) 300, 301 are deposited on the soft magnetic layers 200, 201,respectively. A ferromagnetic layer (island) 100 of Co is also formedbetween the soft magnetic layers 200, 201.

When a voltage V is applied between the soft magnetic layers 200, 201 ofthis tunneling magnetoresistive element, electrons are injected into onesoft magnetic layer 200, tunneling the tunnel barrier layer 300. Thus, acurrent path is formed which reaches the other soft magnetic layer 201through the ferromagnetic layer 100 and tunnel barrier layer 301. Thecurrent along this path of electrons is called tunneling current I. Thissystem is a double junction system since it includes two tunnel barrierlayers 300, 301.

There is another known example 2 (D. V. Averin and Yu. V. Nazarov,“Single Charge Tunneling-Coulomb Blockade Phenomena in Nanostructures,”ed. Hermann Grabert and Michel H. Devoret, Plenum Press, New York, 1992,Chap. 6, pp. 217-247). As described in this example, when theconductance property of the small double junction system of this elementis measured with the bias voltage V fixed to about zero, the classictunneling current I does not appear due to Coulomb blockade effect. Ifthe voltage region in which the classic tunneling current I does notappear is called blockade region, the classic tunneling current I existsout of the blockade region, and is substantially proportional to thevoltage V. In other words, when the bias voltage V is smaller than avoltage Vc (V<Vc) which separates the blockade region from the outside,or when it is within the blockage region, the I-V characteristic of thesmall double junction can be expressed by the following equation.

I∝0   (1)

In addition, when the bias voltage V is larger than the voltage Vc(V>Vc), or when it is out of the blockade region, the I-V characteristicof the small double junction can be expressed by the following equation.

I∝V/(R 1 +R 2)   (2)

Where R1, R2 is the tunnel resistance.

Even within the blockade region, a tunneling current due to thehigher-order term from the viewpoint of quantum mechanics is observed,and this tunneling current I can be expressed by the following equation.

I∝{e ² V ³+(2πkT)² V}/(R 1+R 2)   (3)

Where e is the electric charge of an electron, and k is the Boltzmannconstant.

This tunneling current I is also called current due to co-tunneling. Aswill be seen from the above equation (3), if the voltage V is constant,the tunneling current I is reversely proportional to the product of thetunnel resistances R1, R2. In addition, the tunneling current I includesthe term proportional to the voltage V and the term proportional to thecube of the voltage V.

The current I due to co-tunneling effect is always present within andout of the blockade region. However, out of the blockade region, thecurrent due to co-tunneling effect is negligibly small as compared withthe classic tunneling current. Within the blockade region, since theclassic tunneling current associated with the 0-order term is zero(suppressed), the tunneling current due to co-tunneling effectassociated with high-order terms is chiefly observed.

The tunnel resistances R1, R2 change depending on the direction in whichthe magnetization of the ferromagnetic layer 100 is oriented withrespect to the magnetization of the soft magnetic layers 200, 201. Inother words, the conduction electron spin within the soft magneticlayers 200, 201 of Ni is affected even by an external weak magneticfield, and thus the spin direction is easily changed. The ferromagneticlayer 100 of Co does not easily follow the external weak magnetic field.As a result, the external magnetic field acts to switch the case inwhich the magnetization of electrons within the soft magnetic layers200, 201 and the magnetization within the ferromagnetic layer 100 areparallel and the case in which those are antiparallel to each other.Consequently, the tunneling rate in the path from the soft magneticlayer 200, 201 to the ferromagnetic layer 100 or from the ferromagneticlayer to the soft magnetic layer 200, 201 is changed, so that the tunnelresistances R1, R2 are changed by the variation of the external magneticfield. The effect that the tunnel resistances R1, R2 are changed by theexternal magnetic field is called tunneling magnetoresistance effect. Inthe tunneling magnetoresistive element shown in FIGS. 12A and 12B, achange of tunneling current due to the tunneling magnetoresistanceeffect is observed when the external magnetic field is changed.

According to the above-given equations (1) to (3), the tunneling currentI due to the co-tunneling effect, which is observed within the blockaderegion, is reversely proportional to the product of the tunnelresistances R1 and R2, but the classic tunneling current I observed outof the blockade region is only reversely proportional to the sum of theresistances R1 and R2. Accordingly, when the bias voltage V is constant,the tunneling current within the blockade region is more changed by thevariation of the external magnetic field than that out of the blockaderegion. In other words, the change of the resistance R of the wholesmall double junction specified by the ratio of the voltage V andtunneling current I due to the variation of the magnetic field is moreincreased, or enhanced within the blockade region. That is, the changeof the individual tunnel resistance R1, R2 within the blockade is thesame as out of the blockade, while the resistance R of the whole smalldouble junction is observed to be changed, making the current due to theco-tunneling effect be more changed. This effect can be ascribed to thehigh-order terms of the tunneling phenomenon. Particularly, this effectis formed by a mechanism different from the tunneling magneto-resistanceeffect that is implicitly associated only with 0-order term. However,since this phenomenon due to the co-tunneling effect is due to part ofthe whole tunneling phenomenon, this phenomenon is here called theenhancement of magnetoresistance effect based on the co-tunnelingeffect.

Since the tunneling magnetoresistive element of small double junctiontype shown in FIGS. 12A and 12B has its magnetoresistance effectresulting from the co-tunneling effect, it must be effectively operatedas a single electron device. However, according to the known example 1,the tunneling magnetoresistive element was only operated at a very lowtemperature of about 20 mK. Moreover, according to the known example 1,the size of the ferromagnetic layer 100 is 150 nm×2500 nm. The size ofthe ferromagnetic layer 100 is required to be 5×5 nm or below in orderto operate at room temperature. Thus, since the request for theoperation at room temperature is very difficult to be accepted by theprior art, it is necessary to greatly reduce the size of ferromagneticlayer 100 by a fine working process which is impossible in the priorart.

In addition, the tunneling magnetoresistive element of small doublejunction type has an impedance higher than the conventionalmagnetoresistance (MR) effect element and giant magnetoresistance (GMR)effect element. The reason is that the tunnel resistance is required tobe much larger than the quantum resistance RK (about 25.8 Ω) in order todraw the Coulomb blockade effect. Thus, it is necessary that thetunneling magnetoresistive element be avoided from having the highimpedance. If the tunneling magnetoresistive element cannot be fullyavoided from the high impedance, a signal detection method differentfrom the conventional one is necessary to use in order to improve theS/N ratio.

SUMMARY OF THE INVENTION

It is an object of the invention to provide the multi-tunnelingjunction, tunneling magnetoresistive element, magnetic sensor andmagnetic recording sensor head which can be operated at roomtemperature, thus solving the above problems.

It is another object of the invention to provide a magnetic sensor, andmagnetic recording sensor head which can detect change of tunnelingcurrent at a high S/N ratio.

In order to achieve these objects, according to the present invention, afirst magnetic layer made of one of a soft magnetic material and aferromagnetic material is formed on a flat substrate. First and secondtunnel barrier layers are deposited on the first magnetic layer. Amagnetic particle made of the other one of the soft magnetic materialand the ferromagnetic material is provided between the first and secondtunnel barrier layers. Then, a second magnetic layer made of the one ofthe soft magnetic material and the ferromagnetic material is formed onthe second tunnel barrier layer.

In this case, the capacitance of the magnetic particles is selected tobe 10 aF or below.

The magnetic particle is made a colloidal particle.

Both dielectric material and ferromagnetic material are simultaneouslysputtered as targets to deposit a composite film. Then, the compositefilm is heated to form the first and second tunnel barrier layers andthe magnetic particles.

The magnetic particle is a plurality of particles.

The tunneling magnetoresistive element is formed by themulti-tunneling-junction.

A bias voltage is applied to the magnetic sensor using the tunnelingmagnetoresistive element in order to bring the operating point into theblockade region of the multi-tunneling-junction.

An AC voltage source for applying an AC voltage between the first andsecond magnetic layers is provided in the magnetic sensor using thetunneling magnetoresistive element. In addition, a high-pass filter isprovided in the magnetic sensor in order to produce only the harmoniccomponents including the second harmonic or above of the currentresponse to the AC voltage.

Moreover, the magnetic sensor is used in the magnetic recording sensorhead.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a tunneling magnetoresistive elementhaving a multi-tunneling-junction according to the present invention.

FIGS. 2A and 2B are diagrams to which reference is made in explaining amanufacturing process for the tunneling magnetoresistive element shownin FIG. 1.

FIGS. 3A and 3B are diagrams to which reference is made in explaining amanufacturing process for the tunneling magnetoresistive element shownin FIG. 1.

FIGS. 4A and 4B are diagrams to which reference is made in explaining amanufacturing process for the tunneling magnetoresistive element shownin FIG. 1.

FIG. 5 is a diagram to which reference is made in explaining amanufacturing process for the tunneling magnetoresistive element shownin FIG. 1.

FIGS. 6A and 6B are diagrams to which reference is made in explaining amanufacturing process for the tunneling magnetoresistive element shownin FIG. 1.

FIG. 7 is a cross-sectional view of another tunneling magnetoresistiveelement having a multi-tunneling-junction according to the presentinvention.

FIG. 8 is a cross-sectional view of part of the tunnelingmagnetoresistive element shown in FIG. 7.

FIGS. 9A and 9B are diagrams to which reference is made in explaining amanufacturing process for the tunneling magnetoresistive element shownin FIG. 7.

FIGS. 10A and 10B are diagrams to which reference is made in explaininga manufacturing process for the tunneling magnetoresistive element shownin FIG. 7.

FIG. 11 is a diagram showing a detection circuit system of the magneticsensor according to the invention.

FIGS. 12A and 12B are diagrams showing a conventional tunnelingmagnetoresistive element.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a cross-sectional view of a tunneling magnetoresistive elementhaving multi-tunneling-junction according to the present invention. Asillustrated, a first soft magnetic layer (base electrode) 210 of a softmagnetic material is deposited on a substrate 11 of an insulating layer.An insulating layer 350 is deposited on the substrate 11 and softmagnetic layer 210. First and second tunnel barrier layers 310, 311 of adielectric thin film are formed on the soft magnetic layer 210 in aregion surrounded by the insulating layer 350. Ferromagnetic particles110 of 10 nm or below in particle size are provided between the tunnelbarrier layers 310 and 311. A second soft magnetic layer (top electrode)211 of a soft magnetic material is deposited on the tunnel barrier layer311. Thus, the multi-tunneling-junctions (small double junctions) areformed by the soft magnetic layer 210, tunnel barrier layer 310,ferromagnetic particles 110, tunnel barrier layer 311, and soft magneticlayer 211.

A manufacturing method for the tunneling magnetoresistive element ofmulti-tunneling-junctions shown in FIG. 1 will be described withreference to FIGS. 2 to 6.

First, as shown in FIGS. 2A and 2B (FIG. 2B is a cross-sectional viewtaken along a line A—A in FIG. 2A), the soft magnetic layer 210 isdeposited on the substrate 11 by photolithography and vacuumevaporation. In this case, the substrate 11 may be an SiO₂ film of 400nm or above in thickness formed on an Si substrate by thermal oxidation.The soft magnetic layer 210 may be an NiFe film, permalloy film orFe—Al—Si film of about 10 nm in thickness.

As shown in FIGS. 3A and 3B (FIG. 3B is a cross-sectional view takenalong a line B—B in FIG. 3A), the insulating layer 350 is deposited onthe substrate 11 and soft magnetic layer 210. In this case, theinsulating layer 350 has windows provided for a device creating portion354 at the center and bond pad creating portions 352 on the left andright sides.

As shown in FIGS. 4A and 4B (FIG. 4B is a cross-sectional view takenalong a line C—C in FIG. 4A), the tunnel barrier layer 310 is formedonly in the window for the device creating portion 354. In this case,the tunnel barrier layer 310 may be a dielectric thin film such as analuminum oxide Al—O layer of about 1 nm deposited by sputtering analuminum oxide target. Then, a single-layer of ferromagnetic particles110 of 10 nm or below in particle size is deposited on the surface ofthe tunnel barrier layer 310. In this case, the ferromagnetic particles110 are cobalt particles contained in a colloidal solution preparedaccording to the process described in a known example 3 (J. R. Thomas,“Preparation and Magnetic Properties of Colloidal Cobalt Particles,”Journal of Applied Physics vol. 37, 2914 (1966)). In other words, thesubstrate with the tunnel barrier layer 310 just now formed is immersedinto the colloidal solution which contains cobalt particles of about 8nm in diameter covered by the surfactant of polymer molecules, so thatonly a single layer of ferromagnetic particles 110 (cobalt particles) isdeposited on the tunnel barrier layer 310. The time in which thesubstrate is immersed in the solution is about 24 hours, and during thistime, a static magnetic field H₀ is kept applied to the substrate inparallel to the substrate surface. Thus, the soft magnetic layer 210 inthe static magnetic field H₀ parallel to the substrate is magnetized inthe direction of static magnetic field H₀. However, the surface of thesoft magnetic layer 210 formed by sputtering has the roughness ofmanometer scale as shown in FIG. 5. The soft magnetic material hasoriginally a high permeability. The presence of this roughness causesmagnetic poles 312. Thus, a stray field is distributed in the regionclose to the surface of the soft magnetic layer 210. Since the tunnelbarrier layer 310 is thin enough, the stray field can be considered toleak out on the tunnel barrier layer 310. The ferromagnetic particles110 are magnetized in the direction determined by each crystalorientation. The ferromagnetic particles 110 themselves reach the tunnelbarrier layer 310 while making rotation and Brownian motion. Thus, themagnetic moments of the ferromagnetic particles 110 are not alwaysdirected in a constant direction. However, after arriving on the surfaceof the tunnel barrier layer 310, the ferromagnetic particles 110 areattracted by van der Waals force from the tunnel barrier layer 310, andat the same time, subjected to a force of repulsion from the tunnelbarrier layer 310 by steric hindrance effect of the surfactant moleculeswhich cover the ferromagnetic particles 110 (cobalt particles).Therefore, the ferromagnetic particles 110 are not strongly restrictedto the surface of the tunnel barrier layer 310. The ferromagneticparticles 110 can move around on the surface of the tunnel barrier layer310. When the ferromagnetic particles 110 reach the place where thestray field from the magnetic poles 312 is present while moving aroundon the surface of the tunnel barrier layer 310, the magnetizationdirections of the ferromagnetic particles are made coincident with thedirection of the stray field. Thus, the ferromagnetic particles 110 arefixed on the surface of the tunnel barrier layer 310. It is hereimportant that the magnetization directions of the ferromagneticparticles 110 be kept in parallel to the surface of the tunnel barrierlayer 310 when fixed to the surface of the tunnel barrier layer 310.According to the method of fixing the ferromagnetic particles 110, thedistribution and density of the ferromagnetic particles 110 aredetermined by the distribution of the magnetic poles 312 due to theroughness of the surface of the soft magnetic layer 312. Consequently,the ferromagnetic particles 110 are uniformly deposited over the entiredevice creating portion 354 with a spacing of about 20 mm on theaverage. After the ferromagnetic particles 110 are fixed, the solvent ofthe colloidal solution is removed.

As shown in FIGS. 6A and 6B (FIG. 6B is a cross-sectional view takenalong a line D—D in FIG. 6A), the substrate with the ferromagneticparticles 110 attached is subjected to oxygen plasma ashing, so that thesurfactant molecules are removed from the surface of the ferromagneticparticles 110 (cobalt particles). Then, the tunnel barrier layer 311 isformed by the same method as in the tunnel barrier layer 310. Also, thesoft magnetic layer 211 is deposited. In this case, the soft magneticlayer 211 may be an Ni—Fe film or Fe—Al—Si film of about 10 nm. Leadwires (not shown) are bonded to the soft magnetic layers 210 and 211,and the device is connected to the external circuit through the leadwires.

Here, the size of the ferromagnetic particles 110 is the importantparameter. In other words, most particles of ferromagnetic materialslose coercive force and exhibit super paramagnetism at room temperaturewhen their diameter is 3 nm or below. The metal particles of 10 nm orbelow have a capacitance of about 1 aF (10⁻¹⁸ F). The charging energycalculated from this capacitance is about 100 meV, which is much largerthan the thermal exciting energy 25 meV at room temperature. Therefore,in order to obtain the Coulomb blockade effect at room temperature, itis necessary to make the diameter of the ferromagnetic particles 110equal to or less than 10 nm. Thus, for the purpose of maintainingcoercive force, it is required to use ferromagnetic particles 110 of acertain size or above, and for observing the Coulomb blockade effect atroom temperature, it is necessary to use the ferromagnetic particles ofmuch small size. That is, the size of the ferromagnetic particles 110must be selected to satisfy both conditions.

In this tunneling magnetoresistive element, a multi-tunneling-junctionis formed through each of the ferromagnetic particles 110 interposedbetween the tunnel barrier layers 310, 311 which are formed between thesoft magnetic layers 210, 211. Thus, since there are a large number offerromagnetic particles 110, many multi-tunneling-junctions is parallelconnect the soft magnetic layers 210, 211.

In addition, considering only one of many multi-tunneling-junctionspresent between the soft magnetic layers 210, 211, it is assumed that apositive voltage V relative to the soft magnetic layer 210 is applied tothe soft magnetic layer 211. In this case, it is important if thisvoltage is the voltage within or out of the blockade region which themulti-tunneling-junction of interest has. If this voltage is out of theblockade region, electrons are injected into the ferromagnetic particle110 through the tunnel passing the tunnel barrier layer 310 from thesoft magnetic layer 210. Then, electrons are discharged from theferromagnetic particles 110 to the soft magnetic layer 211 through thetunnel passing the tunnel barrier layer 311. Chiefly, single electrontunnel current formed of classic tunnel current flows. If the voltage Vis within the blockade region, the classic tunnel current is suppressed,and chiefly the tunnel current due to co-tunneling effect is observed.

In addition, the soft magnetic layers 21, 211 can be easily magnetizedto be inverted by an external field parallel to the film surface. Theferromagnetic particles 110 do not easily follow the external fieldsince they have fixed magnetization in the crystal structure. Themagnetizations of the ferromagnetic particles 110, and soft magneticlayers 210, 211 (the magnetizations of each magnetic layer) have adifferent response to the external field, thus giving rise to thetunneling magnetoresistance effect. In other words, as the ferromagneticparticles 110 and soft magnetic layers 21, 211 change theirmagnetization directions, the tunnel resistances R1, R2 of the tunnelbarrier layers 310, 311 change according to the change of themagnetization direction. As a result, the tunnel current I flowing inthe multi-tunneling-junctions changes. The tunneling magnetoresistanceeffect of multi-tunneling-junction depends on whether the bias voltage Vis within or out of the blockade region. If it is within the blockaderegion, the effect is enhanced by co-tunneling. In the tunnelingmagnetoresistive element of multi-tunneling-junction shown in FIG. 1,since the diameter of the ferromagnetic particles is 10 nm or below, aremarkable Coulomb blockade effect can be produced at room temperature.If a voltage within the blockade region is applied at room temperature,a tunnel current due to co-tunneling effect is observed. Thus, thetunnel resistances R1, R2 can be detected. Therefore, the tunnelingmagnetoresistive element can be operated at room temperature.

The above description is about one multi-tunneling-junction. Since agreat number of ferromagnetic particles 110 are present between the softmagnetic layers 210, 211, many multi-tunneling-junctions are formed. Theferromagnetic particles 110 have a uniform diameter. In addition, ifboth tunnel barrier layers 310, 311 have a uniform film thickness, thecharacteristics of a large number of multi-tunneling-junctions areuniform. As a result, a great number of multi-tunneling-junctions withuniform characteristics are connected in parallel between the softmagnetic layers 210, 211. Moreover, since the ferromagnetic particles110 are provided with a spacing of about 20 nm, the mutual action amongthe multi-tunneling-junctions can be neglected, or the individualmulti-tunneling-junctions are independently operated. Therefore, theobserved resistance values between the soft magnetic layers 210, 211correspond to the parallel connection of those independentmulti-tunneling-junctions of uniform characteristics. If the windowregion of the device creating portion 354 is of a square shape of about2 μm for each side, the number of multi-tunneling-junctions is 10000 orabove. The observed resistance value between the soft magnetic layers210, 211 is reduced to about {fraction (1/1000)} that of a singlemulti-tunneling-junction. In addition, since the characteristics of themulti-tunneling-junctions are uniform, the tunneling magnetoresistanceeffect due to co-tunneling effect at all multi-tunneling-junctions canbe enhanced because the bias voltage within the blockade region isapplied to the other multi-tunneling-junctions if the voltage V to acertain multi-tunneling-junction is set within the blockade region.

FIG. 7 is a cross-sectional view of another tunneling magnetoresistiveelement having multi-tunneling-junction according to the invention. FIG.8 is a detailed diagram of part of the structure shown in FIG. 7. Asillustrated, the soft magnetic layer 210 is deposited on the substrate11 of an insulating layer. A matrix 125 of aluminum oxide and of about 8nm in thickness b is formed on the soft magnetic layer 210. In thematrix 125, there are dispersively provided cobalt particles 120 ofabout 6 nm in diameter a as magnetic particles. The soft magnetic layer211 is deposited on the matrix 125.

A manufacturing process for the tunneling magnetoresistive element shownin FIGS. 7 and 8 will be described with reference to FIGS. 9A, 9B, 10Aand 10B. First, as shown in FIGS. 9A and 9B (FIG. 9A is across-sectional view taken along a line E—E in FIG. 9A), the softmagnetic layer 210 is deposited on the substrate 11 by photolithographyand vacuum evaporation. Then, aluminum oxide as a dielectric materialand cobalt as a ferromagnetic material are simultaneously sputtered astargets to form a composite film of about 8 nm in thickness on the softmagnetic layer 210. A pattern of composite film is formed bylithography, and then the composite is heated to form the matrix 125with cobalt particles 120 dispersed. As shown in FIGS. 10A and 10B (FIG.10B is a cross-sectional view taken along a line F—F in FIG. 10A), thesoft magnetic layer 211 having a pattern is formed by sputtering andphotolithography. Thereafter, lead wires (not shown) are bonded to thesoft magnetic layers 210, 211, and this device is connected to anexternal circuit.

In the tunneling magnetoresistive element of multi-tunneling-junctionshown in FIG. 7, the surfaces of the cobalt particles 120 are separatedabout 1 nm from the soft magnetic layer 210 or 211. Thus, the matrix 125present between the cobalt particle 120 and the soft magnetic particle210 or 211 acts as a first or second tunnel barrier layer, making itpossible for electrons to tunnel from the soft magnetic layer 210, 211to the cobalt particles 120. In addition, since the thickness b of thematrix 125 is about 8 mm, only a single layer of cobalt particles 120can exist between the soft magnetic layers 210, 211. Thus, since thepath of tunnel to the cobalt particles 120 is only the path to the softmagnetic layer 210, 211, the multi-tunneling-junctions are formed. Inaddition, since the cobalt particles 120 are deposited at intervals ofabout 20 nm on the average, the individual multi-tunneling-junctions canbe considered to independently operate. In the tunnelingmagnetoresistive element of multi-tunneling-junction shown in FIG. 7, itis possible to easily form the first and second tunnel barrier layers ofmatrix 125, and the magnetic particles of cobalt particles 120.

As a dielectric material, silicon dioxide SiO₂ can be used instead ofaluminum oxide. As a ferromagnetic material, iron or other alloys can beused in place of cobalt.

The tunneling magnetoresistive elements shown in FIGS. 1 and 7 can beoperated as magnetic sensors by applying the voltage V associated withthe inside of the blockade region of multi-tunneling-junction to thesoft magnetic layers 210, 211, and monitoring the change of tunnelcurrent I to the magnetic field. They can also be operated as magneticsensors by fixing the tunnel current I and monitoring the voltage Vdeveloped between the soft magnetic layers 210, 211. In this case, it isnecessary to set the current bias I so that the generated voltage Vstays within the blockade region. These magnetic sensors can also beused for magnetic recording sensor heads. In either case, the change ofmagnetic field is detected as the change of resistance R between thesoft magnetic layers 210, 211. According to the experiments by theinventors, the observed resistance value R across the terminals wasabout several hundred kΩ. In addition, the resistance R across theterminals was the maximum in the magnetic filed of about 100 Oe. Thedifference ΔR (═R—Rm) between the resistance R across the terminals andthe minimum resistance Rm across the terminals in the external field of1 kOe or above was about 100 kΩ. The ratio of the difference ΔR to theminimum resistance Rm across the terminals, ΔR/Rm reached 40%. Thechange of the voltage V across the terminals was about 20 μV.

Let us consider that the relation between the external field and thetunnel current I is measured when a higher voltage V out of the blockaderegion is applied to the magnetic sensor using the tunnelingmagnetoresistive element shown in FIG. 1 or 7. As is evident from theequation (3), the tunnel current I includes two components: a componentproportional to the voltage V, and a component proportional to the cubeof the voltage V. In addition, the classic tunnel current, and thecurrent of electrons passing through the element across the tunnelbarrier layers by thermal excitation all include only the componentproportional to the voltage V. Therefore, if only the componentproportional to the cube of the voltage V can be detected even under theapplication of a higher voltage out of the blockade region, it ispossible to detect only the component due to the co-tunneling effect, orthe enhanced tunneling magnetoresistance effect, and to remove the othercurrent components not enhanced.

FIG. 11 is a diagram showing a detection circuit system of a magneticsensor according to the invention. As illustrated, an AC voltage source500 of frequency f and voltage V (f) is connected to the soft magneticlayer 211, a current detection amplifier 510 connected to the softmagnetic layer 210, and a high-pass filter 520 to the current detectionamplifier 510.

In the detection circuit system of this magnetic sensor, the tunnelcurrent I (f, 3f) (element current) changes when an external field H ischanged. The tunnel current I (f, 3f) contains a component of frequency3f due to the term proportional to the cube of the voltage V, and acomponent of frequency f. However, the output from the current detectionamplifier 510 is supplied to the high-pass filter 520, and a signal V₀(3f) having only the frequency 3f component is produced by the frequencydiscrimination. Thus, the circuit system removes the frequency fcomponent containing the signal component in which the tunnelingmagnetoresistance effect is not enhanced. In addition, if the responseof the frequency 3f component to the change of external field H istreated as a sensor signal, a larger signal component is generated bythe higher voltage V out of the blockade region, since the componentproportional to the cube of the voltage V suddenly increases with theincrease of voltage V. Therefore, the voltage V to be applied to thesoft magnetic layers 210, 211 is not necessary to be limited to theinside of the blockade region, and the change of the external field Hcan be measured at a high S/N ratio raised by about three figures. Thismagnetic sensor can also be used for the magnetic recording sensor head.

While the above embodiments employ the soft magnetic layers 210, 211 forthe first and second magnetic layers, and the ferromagnetic particles110 of a ferromagnetic material for magnetic particles, it is possibleto employ first and second ferromagnetic layers of a ferromagneticmaterial for the first and second magnetic layers, and soft magneticparticles for the magnetic particles. In this case, the soft magneticparticles follow the external field, and the first and secondferromagnetic layers have fixed magnetization to the crystal structure.In addition, while the above embodiments select the capacitance of theferromagnetic particles 110 to be about 1 aF, the capacitance of themagnetic particles is selected to be 10 aF or below.

In the tunneling magnetoresistive element of multi-tunneling-junctionaccording to the invention, the Coulomb blockade effect can be attainedat room temperature, and thus if a voltage within the blockade region isapplied at room temperature, the tunnel resistance can be detected byobserving the tunnel current due to the co-tunneling effect. Thus, thetunneling magnetoresistive element can operate at room temperature.

Moreover, the first and second tunnel barrier layers and magneticparticles can be easily formed by simultaneous sputtering of adielectric material and a magnetic material as targets and heating thecomposite film deposited by the sputtering.

In addition, by providing a plurality of magnetic particles, it ispossible to reduce the resistance between the first and second magneticlayers.

Also, the magnetic sensor and magnetic sensor head can be operated atroom temperature by applying a bias voltage within the blockade regionof multi-tunneling-junction.

In addition, the change of the tunnel current due to the magnetic fieldcan be detected at a high S/N ratio when the magnetic sensor or magneticrecording sensor head has an AC voltage source provided for applying anAC voltage between the first and second magnetic layers, and a high-passfilter provided for taking out only the harmonic signal componentcontaining frequencies of the second harmonic or above of the currentresponse to the AC voltage.

What is claimed:
 1. A magnetic head comprising a tunnelingmagnetoresistive element having a first magnetic layer of a softmagnetic material, first and second tunnel barrier layers formed on saidfirst magnetic layer, a magnetic particle of a ferromagnetic materialprovided between the first and second tunnel barrier layers, and asecond magnetic layer of a soft magnetic material formed on said secondtunnel barrier layer so as to create tunneling junctions, said tunnelingmagnetoresistive element having a current flow which changes accordingto the change of an external magnetic field, the external filed beingdetected based on the voltage between said first and second magneticlayers.
 2. The magnetic head according to claim 1 and further includingan ac voltage source coupled to said first magnetic layer and a currentdetection system coupled to said second magnetic layer.
 3. The magnetichead according to claim 2 wherein said current detection system includesa current detection amplifier and a high pass filter coupled to the output of said amplifier to produce a voltage output from said filterrepresenting the voltage between said first and second magnetic layers.4. A tunneling magnetoresistive element according to claim 1, whereinthe capacitance of said magnetic particle is selected to be 10 pF orbelow.
 5. A tunneling magnetoresistive element according to claim 1,wherein said magnetic particle is provided between said first and secondtunnel barrier layers using a colloidal solution.
 6. A tunnelingmagnetoresistive element according to claim 1, wherein said first andsecond tunnel barrier layers and said magnetic particles are formed bysimultaneously sputtering a dielectric material and a ferromagneticmaterial as targets to deposit a composite film and heating saidcomposite film.
 7. A tunneling magnetoresistive element according toclaim 1, further including a plurality of magnetic particles whereinsaid magnetic particle is one of said plurality of magnetic particles.8. A tunneling magnetoresistive element having a first magnetic layer ofa ferromagnetic material formed on a flat substrate, first and secondtunnel barrier layers formed on said first magnetic layer, a magneticparticle of a soft magnetic material provided between the first andsecond tunnel barrier layers, and a second magnetic layer of aferromagnetic material formed on said second tunnel barrier layer so asto create tunneling junctions.
 9. A tunneling magnetoresistive elementaccording to claim 6, wherein the capacitance of said magnetic particleis selected to be 10 aF or below.
 10. A tunneling magnetoresistiveelement according to claim 6, wherein said magnetic particle is providedbetween said first and second tunnel barrier layers using a colloidalsolution.
 11. A tunneling magnetoresistive element according to claim 6,further including a plurality of magnetic particles wherein saidmagnetic particle is one of said plurality of magnetic particles.